1. Field of the Invention
The present invention relates to electrical interfaces and particularly to thermally conductive interfaces for use in a variety of electronic products.
2. Description of Related Art
Integrated circuit ("IC") chips are steadily become more powerful while being compacted into smaller and smaller packages. When compared to previous integrated circuit chips, this trend produces integrated chips which are significantly denser and which perform many more functions in a given period of time--resulting in an increase in the current they use. Consequently, smaller and faster chips tend to run significantly hotter than previous products.
As a result, heat management in electronic products has become a chief concern in product design. Reliability of electronic circuits tends to be tied to proper matches in the coefficients of expansion of various electronic components. As the temperature rises, mismatches in the coefficients of expansion cause stresses to develop between adjoining members. Under these circumstances, any increase in operating temperature will have a negative effect on reliability.
In an effort to control heat better, the use of various heat sinks is now a central focus in electronic equipment design. Examples of common heat sinks employed today include: various filled products, such as epoxies, thermosets, silicones, and thermoplastics; IBM Thermal Conductive Modules (ITCM); Mitsubishi High Thermal Conduction Modules (HTCM); Hitachi SiC Heat Sink; Fujitsu FACOM VP2000 Cooling Mechanism; etc.
In order to mate IC chips to heat sinks successfully, a interface which is elastic or otherwise conformable is preferred so as to ease installation and to minimize the effect of expansion and contraction between electronic components. Air gaps formed from inapt installation of a chip to a heat sink, and/or expansion and contraction cycles during operation, can greatly impede the flow of heat from the device. Conformability becomes especially important when the tolerances on the heat sink and chip tilt (in the case of flip chips) become large.
Typically, thermal greases or thermally conductive thermosetting materials are used to take up tolerances between electronic components. See, e.g., U.S. Pat. No. 5,028,984 to Ameen et al. While such materials may work well for some applications, they continue to have a number of drawbacks. These materials tend to be hard to control and are prone to contaminating components of the electronic device. For instance, care must be taken when using these materials to prevent unwanted contamination of solder joints and, in the case of electrically conductive thermoset resins, unwanted contamination of adjacent conductors. In practice, this usually results in a significant amount of wasted material. Additionally, clean up often requires the use of either unsafe or environmentally unsound solvents.
In U.S. Pat. No. 5,187,283 to Giarusso et al. a gasket-type material is disclosed comprising a thin-film surrounding a meltable metal core. In operation, the gasket is installed as an interface and its temperature is increased to melt the metal core and allow it to conform to the component parts. Unfortunately, this construction is believed to be ineffective in avoiding air gaps that can form during normal thermal cycling of the device. Further, as is a common problem with solid gasket materials in general, it is believed that this device may experience limited compressibility, requiring either application of excessive pressure to the mating surfaces, or use of unacceptably thick sections of the gasket.
In U.S. Pat. No. 5,060,114 to Feinberg et al., conformability is sought by curing a metal or metal oxide filled silicone around the component to be cooled. Although this method may be successful, it is believed to be unduly complicated, costly, and time consuming for practical widespread use.
In addition, with most thermoset resins, greases, and gaskets employing a filler there are additional constraints in successful heat dissipation. Most fillers tend to coat each individual particle of the thermal conductor within the resin--essentially insulating the conductor. This greatly reduces the overall effective thermal conductivity of the product in at least two ways. First, even a thinly coated surface (e.g., with a layer of silicone or epoxy) can serve as a thermal insulator, reducing the effective thermal conductivity of the product, particularly at contacting surfaces. Second, in order to overcome such thermal insulation, it is often necessary to apply substantial pressure to the interface in order to urge the thermally conductive particles into direct contact with one another to produce the necessary amount of conduction through the material. This often requires unacceptable compressive force for integrated circuits to produce a viable thermally conductive interface.
As a result, most commercially available products can produce a conductivity in the range of only about 1.8 W/M .degree.K. (for greases) to 2.2 W/M .degree.K. (for epoxies). Even the most advanced (and expensive) materials, such as silver filled epoxies, can achieve a conductivity in the range of 3-4 W/M .degree.K. With regard to easily handled materials, such as self-adhesive materials available from Chomerics, Inc., Woburn, Mass., under the trademark CHO-THERM Thermal Interface Materials, and from The Bergquist Company, Minneapolis, Minn., under the trademark SIL-PAD Thermal Management Materials, these can typically achieve a conductivity of only about 0.37-0.95 W/M .degree.K. and 0.6-1.5 W/M .degree.K., respectively. Although these commercial materials can produce better conductivities at high mounting pressures, they deliver extremely poor conductivity at low mounting pressures (e.g., pressures below 2-3 lbs/in.sup.2).
Other problems experienced by many commercially available thermoset resins include: inadequate conformability (i.e., excessive compressive force to get higher thermal conductivity); high flexural modulus after curing--resulting in substantial stress upon devices during thermal cycling; a lack of "compliance," resulting in stress fractures if the resin is flexed longitudinally after curing; long curing times; and difficulty in manufacturing in high volumes.
Accordingly, it is a primary purpose of the present invention to provide a thermally conductive interface which delivers relatively even heat dissipation and reduces the negative impact of flex and fatigue.
It is another purpose of the present invention to provide a thermally conductive interface which is conformable to provide a good fit between component parts without requiring undue compressive force to achieve the desired amount of thermal conductivity.
It is still another purpose of the present invention to provide a thermally conductive interface which is compliant, allowing the material to be more forgiving to longitudinal stresses.
These and other purposes of the present invention will become evident from review of the following specification.